GaN MISHEMTs in RF PAs Show Stable Operating Range

GaN MISHEMTs are currently under investigation for their potential application in advanced 5G+/6G RF systems, thanks to their remarkable efficiency and power-handling capabilities. However, these devices face certain challenges, particularly in relation to positive gate bias instability. This instability can lead to shifts in the threshold voltage under specific conditions, potentially impacting the performance and long-term reliability of power amplifiers.

At the IEEE International Reliability Physics Symposium (IRPS) in 2025, imec made a significant breakthrough by demonstrating that GaN MISHEMTs can maintain consistent performance despite their positive bias instability when operated within a defined range. This breakthrough opens up possibilities for designing reliable GaN-based power amplifiers, which could be crucial for handset applications in future 6G communication systems.

The phenomenon of gate bias instability in GaN MISHEMTs is complex and not yet fully understood. It can manifest in different operational states—off, semi-on, and on state—each with its distinct instability mechanisms. With the evolving requirements of wireless standards, GaN MISHEMTs are becoming an attractive option, especially as traditional RF power amplifiers predominantly use GaAs HBT or HEMTs without a dielectric gate.

Imec researchers introduced an analytical approach that compared stable gate voltages in DC conditions with the gate modulation range during RF power amplifier operation. Their analysis revealed a significant overlap between these ranges, indicating that GaN MISHEMTs can remain stable within the typical voltage swing of RF power amplifiers. This finding allows for the design of linearly operating power amplifiers that can effectively mitigate positive gate bias instability.

Hao Yu, Principal Member of the Technical Staff at imec, emphasized the practical implications of their research, stating, “Our work not only identifies challenges but also provides solutions, demonstrating the reliable use of GaN MISHEMTs in power amplifier applications for 5G+/6G technology. By combining fundamental device reliability research with real-world RF system assessments, our team at imec is bridging the gap between theory and practical applications, ensuring that GaN technology meets the demands of next-generation communication systems.”

Furthermore, the researchers highlighted the role of naturally occurring positive interfacial polarisation charges at the material interface in preventing unwanted shifts in operating voltage over time. This unique characteristic of GaN MISHEMTs makes them more resilient to on-state threshold voltage instability compared to other gate dielectric-contained devices. Simulations also demonstrated that even under real-world RF input signals, the instability in on-state threshold voltage can be effectively mitigated.

At the 2025 IEEE IRPS conference, imec presented their research alongside three additional papers from their advanced RF program, developed in collaboration with Prof. Tian-Li Wu’s team at National Yang-Ming Chiao Tung University (NYCU) in Taiwan. These papers delve into fundamental research on different degradation mechanisms and reliability challenges of GaN HEMTs and MISHEMTs under various stress conditions, providing a comprehensive understanding of GaN-Si device performance.

Image: Simulated probability profiles illustrating the operational range of a power amplifier. The region where Nch<NFB represents the safe operating range of a MISHEMT PA without concerns of on-state threshold voltage instability.